The following discussion is not to be construed as an admission with regard to the state of the common general knowledge.
Synthetic polymeric membranes are well known in the field of ultrafiltration and microfiltration for a variety of applications including desalination, gas separation, filtration and dialysis. The properties of the membranes vary depending on the morphology of the membrane i.e. properties such as symmetry, pore shape, pore size and the chemical nature of the polymeric material used to form the membrane.
Different membranes can be used for specific separation processes, including microfiltration, ultrafiltration and reverse osmosis. Microfiltration and ultrafiltration are pressure driven processes and are distinguished by the size of the particle or molecule that the membrane is capable of retaining or passing. Microfiltration can remove very fine colloidal particles in the micrometer and submicrometer range. As a general rule, microfiltration can filter particles down to 0.05 μm, whereas ultrafiltration can retain particles as small as 0.01 μm and smaller. Reverse Osmosis operates on an even smaller scale.
Microporous phase inversion membranes are particularly well suited to the application of removal of viruses and bacteria.
A large surface area is needed when a large filtrate flow is required. A commonly used technique to minimize the size of the apparatus used is to form a membrane in the shape of a hollow porous fibre. A large number of these hollow fibres (up to several thousand) are bundled together and housed in modules. The fibres act in parallel to filter a solution for purification, generally water, which flows in contact with the outer surface of all the fibres in the module. By applying pressure, the water is forced into the central channel, or lumen, of each of the fibres while the microcontaminants remain trapped outside the fibres. The filtered water collects inside the fibres and is drawn off through the ends.
The fibre module configuration is a highly desirable one as it enables the modules to achieve a very high surface area per unit volume.
In addition to the arrangement of fibres in a module, it is also necessary for the polymeric fibres themselves to possess the appropriate microstructure to allow microfiltration to occur.
Desirably, the microstructure of ultrafiltration and microfiltration membranes is asymmetric, that is, the pore size gradient across the membrane is not homogeneous, but rather varies in relation to the cross-sectional distance within the membrane. Hollow fibre membranes are preferably asymmetric membranes possessing tightly bunched small pores on one or both outer surfaces and larger more open pores towards the inside edge of the membrane wall.
This microstructure has been found to be advantageous as it provides a good balance between mechanical strength and filtration efficiency.
As well as the microstructure, the chemical properties of the membrane are also important. The hydrophilic or hydrophobic nature of a membrane is one such important property.
Hydrophobic surfaces are defined as “water hating” and hydrophilic surfaces as “water loving”. Many of the polymers used to cast porous membranes are hydrophobic polymers. Water can be forced through a hydrophobic membrane by use of sufficient pressure, but the pressure needed is very high (150-300 psi), and a membrane may be damaged at such pressures and generally does not become wetted evenly.
Hydrophobic microporous membranes are typically characterised by their excellent chemical resistance, biocompatibility, low swelling and good separation performance. Thus, when used in water filtration applications, hydrophobic membranes need to be hydrophilised or “wet out” to allow water permeation.
It is also important that membranes have a high resistance to aggressive chemical species typically found in water requiring filtration, in particular, to oxidising agents and conditions of high pH (i.e. caustic solutions). In particular with water filtration membranes, chlorine resistance is highly desirable. Chlorine is used to kill bacteria and is invariably present in town water supplies. Even at low concentrations, a high throughput of chlorinated water can expose membranes to large amounts of chlorine over the working life of a membrane can lead to yellowing or brittleness which are signs of degradation of the membrane.
Currently, poly(tetrafluoroethylene) (PTFE), polyethylene (PE), polypropylene (PP) and poly(vinylidene fluoride) (PVDF) are the most popular and available hydrophobic membrane materials. However, the search for membrane materials which will provide better chemical stability and performance while retaining the desired physical properties required to allow the membranes to be formed and worked in an appropriate manner has suggested that halogentated polymers may be suitable. In particular, Halar ultrafiltration (UF) membranes have been found to be superior in nearly every way to any unsupported hollow-fibre UF membrane on the market.
Microporous synthetic membranes are particularly suitable for use in hollow fibres and are produced by phase inversion. In this process, at least one polymer is dissolved in an appropriate solvent and a suitable viscosity of the solution is achieved. The polymer solution can be cast as a film or hollow fibre, and then immersed in precipitation bath such as water. This causes separation of the homogeneous polymer solution into a solid polymer and liquid solvent phase. The precipitated polymer forms a porous structure containing a network of uniform pores. Production parameters that affect the membrane structure and properties include the polymer concentration, the precipitation media and temperature and the amount of solvent and non-solvent in the polymer solution. These factors can be varied to produce microporous membranes with a large range of pore sizes (from less than 0.1 to 20 μm), and possess a variety of chemical, thermal and mechanical properties.
Hollow fibre ultrafiltration and microfiltration membranes are generally produced by either diffusion induced phase separation (the DIPS process) or by thermally induced phase separation (the TIPS process).
The TIPS process is described in more detail in PCT AU94/00198 (WO 94/17204) AU 653528, the contents of which are incorporated herein by reference.
The quickest procedure for forming a microporous system is thermal precipitation of a two component mixture, in which the solution is formed by dissolving a thermoplastic polymer in a solvent which will dissolve the polymer at an elevated temperature but will not do so at lower temperatures. Such a solvent is often called a latent solvent for the polymer. The solution is cooled and, at a specific temperature which depends upon the rate of cooling, phase separation occurs and the polymer rich phase separates from the solvent.
The term “solvent” as used herein will be understood by those in the art to encompass: single component mixtures and multiple component mixtures. Multiple component mixtures may include, in addition to solvent components, one or more non-solvents.
In the present case the inventors have sought to find a way to modify polymers and polymeric membranes made from halogenated polymers to enhance the range of applications in which they may be used, while at the same time, retaining the good intrinsic resistance of the material to chemical, physical and mechanical degradation. One such polymer is halar.
Halar, or poly (ethylene chlorotrifluoroethylene), is a 1:1 alternating copolymer of ethylene and chlorotrifluoroethylene with the following structure:—(—CH2—CH2—CFCl—CF2—)n—
Halar and related membranes have extremely good chemical resistance to species such as chlorine, peroxide and ozone, and are stable over a wide range of pHs (0-14)
Processes used to produce flat sheet Halar membranes are disclosed for example in U.S. Pat. No. 4,702,836. The properties of Halar make it highly desirable in the field of ultrafiltration and microfiltration. In particular, Halar has extremely good properties in relation to its resistance both to chlorine and to caustic solutions, but also to ozone and other strong oxidising agents.
Halar membranes also have good mechanical and structural properties. Halar produces membranes of near-perfect sub-structure, with little or no defects or macrovoids. Good permeabilities, in the range of 300-1000 lmh/bar can be achieved. Halar membranes, particularly hollow fibre membranes, have a good break extension, of greater than 100%, a break force in excess of 2N and exhibit high flexibility with little or no brittleness.
As a result of this good integrity, Halar membranes have been able to achieve log virus retentions (LRV) of ≧4, ie better than 1 in 10,000 viral particles removed.
However, Halar and related membranes are not without some drawbacks. They have a tendency to exhibit reasonable performance initially, but after a relatively short time in use, can suffer from irreversible fouling, pore-compaction or both.
In the present case the inventors have sought to find a way to modify polymers and polymeric membranes made from halogenated polymer such as Halar to enhance the range of applications in which they may be used, while at the same time, retaining the good intrinsic resistance of the material to chemical, physical and mechanical degradation. The most desirable modification is to render the material hydrophilic.
Hydrophilisation of membranes with agents such as PVP has been attempted previously. U.S. Pat. Nos. 5,376,274 and 5,629,084 both disclose coating a polysulfone membrane with a PVP/initiator (persulfate) solution and then heating the soaked membrane to crosslink the PVP. This was successful in converting the hydrophobic membrane into a hydrophilic one to improve the wettability of the membrane for filtering aqueous solutions. However, the treatment is of a very short term nature and only a minor increase in the time to irreversible fouling is realised.
Halar, because of its very inertness, is not readily amendable to functionalisation or chemical modification, and for this reason, has not been used as widely as some other membrane forming polymers which are less resistant to environmental degradation. Attempts to hydrophilise Halar in the past have proved difficult for this very reason.
One approach to hydrophilising halar membranes carried out by the present applicant has been to remove HCl from the polymer by exposure to aqueous solutions of alkali earth hydroxides or alkoxides, such as caustic soda or sodium methoxide to produce an activated form of halar possessing double bonds at the surface of the polymer. The activated halar is then treated with an oxidising agent, such Fenton's reagent, which acts as a source of hydroxyl radicals. The hydroxyl radicals react with the double bonds to produce a hydroxylated form of halar which is more hydrophilic than unmodified halar. This is disclosed in AU 2004903680, the contents of which is incorporated by reference in its entirety in the present application. This hydroxylated form of halar is also more amenable to reaction with other chemical species. However, alternative approaches for the long term hydrophilisation of hydrophobic membranes are still desirable.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative, particularly in terms of methods of production.
Unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.